Electroactive polymers (EAPs) are promising candidate materials for the design of drug delivery technologies, especially in conditions where an "on-off" drug release mechanism is required. To achieve this, EAPs such as polyaniline, polypyrrole, polythiophene, ethylene vinyl acetate, and polyethylene may be blended into responsive hydrogels in conjunction with the desired drug to obtain a patient-controlled drug release system. The "on-off" drug release mechanism can be achieved through the environmental-responsive nature of the interpenetrating hydrogel-EAP complex via (i) charged ions initiated diffusion of drug molecules; (ii) conformational changes that occur during redox switching of EAPs; or (iii) electroerosion. These release mechanisms are not exhaustive and new release mechanisms are still under investigation. Therefore, this review seeks to provide a concise incursion and critical overview of EAPs and responsive hydrogels as a strategy for advanced drug delivery, for example, controlled release of neurotransmitters, sulfosalicyclic acid from cross-linked hydrogel, and vaccine delivery. The review further discusses techniques such as linear sweep voltammetry, cyclic voltammetry, impedance spectroscopy, and chronoamperometry for the determination of the redox capability of EAPs. The future implications of the hydrogel-EAP composites include, but not limited to, application toward biosensors, DNA hybridizations, microsurgical tools, and miniature bioreactors and may be utilized to their full potential in the form of injectable devices as nanorobots or nanobiosensors.

Electro-conductive hydrogels based on poly(vinyl alcohol), crosslinked with diethyl acetamidomalonate as the hydrogel component, were engineered using polyaniline as the inherently conductive component, and fabricated in the form of cylindrical devices to confer electro-actuable release of the model drug indomethacin. The hydrogels were characterized for their physicochemical and physicomechanical properties. Cyclic voltammetry was employed for electro-activity and conductivity analysis. Drug entrapment efficiency ranged from 65-70%. "ON-OFF" switchable drug release was obtained by periodically applying-removing-reapplying an electric potential ranging from 0.3-5.0 V for 60 seconds at hourly intervals and the cumulative drug release obtained ranged from 4.7-25.2% after four release cycles respectively. The electro-stimulated release of indomethacin was associated with the degree of crosslinking, the polymeric ratio and drug content. A Box-Behnken experimental design was constructed employing 1.2 V as the baseline potential difference. The devices demonstrated superior swellability and high OPEN ACCESSPolymers 2011, 3 151 diffusivity of indomethacin, in addition to high electrical conductivity with "ON-OFF" drug release kinetics via electrical switching. In order to investigate the electro-actuable release of indomethacin, molecular mechanics simulations using AMBER-force field were performed on systems containing water molecules and the poly(vinyl alcohol)-polyaniline composite under the influence of an external electric field. Various interaction energies were monitored to visualize the effect of the external electric field on the erosion of polyaniline from the co-polymeric matrix. This strategy allows the electro-conductive hydrogels to be suitably applied for controlled, local and electro-actuable drug release while sustaining a mild operating environment.